The production of grain oriented electrical steel is based on the metallurgical phenomenon called secondary recrystallisation, in which a primary recrystallised strip undergoes after cold deformation an annealing in which, by means of a slow heating, it is brought up to about 1200° C. During this heating, at a temperature comprised between 900 and 1100° C. the grains having an orientation close to {110} <001> (Goss grains), which in the primary recrystallised strip are a minority, abnormally grow at the expenses of the other crystals, to become the only grains present in the microstructure, with macroscopic dimensions (5–20 mm).
The mechanism on which the secondary recrystallisation is based is rather complex. The experts agree that secondary recrystallisation is the result of a delicate equilibrium among three factors: the mean diameter of the primary grain (governing the attitude of the crystals to grow), the texture of the strip, in a decarburised state (which can constitute a small advantage in the growth of the Goss crystals) and the presence of evenly distributed fine second phases (which, slowing down the tendency to grow of all the crystals, lets the Goss grains, present as a minority in the primary recrystallised strip, to acquire a dimensional advantage. Thus, at the higher temperatures of 900–1100° C. at which second phases are dissolved into the matrix thus permitting the grains to freely grow, the Goss grains, slightly larger than the other, can rapidly grow at the expenses of the latter.
In the traditional technologies for the production of grain oriented Fe-3% Si (Takahashi, Harase: Mat. Sci. Forum Voll. 204–206 (1996) pp 143–154; Fortunati, Cicalé, Abbruzzese: Proc. 3rd Int. Conf. On Grain Growth, TMS Publ. 1998, p. 409), necessary microstructure and texture of the product are obtained by means of a process requiring the following sequence of steps: slab casting, hot rolling, cold rolling, recrystallisation annealing. The desired distribution of second phases is obtained by heating the stab at high temperature (>1350° C.) to dissolve them, and re-precipitating the same in fine form during the hot rolling step and during the subsequent annealing of the hot rolled strip.
The second phases usually utilised as grain growth inhibitors are substantially of two kinds: (i) sulphides and/or selenides of manganese, copper or mixtures thereof, and (ii) aluminium nitrides, alone or in combination with the above sulphides and/or selenides.
In the state of the art for the production of grain oriented electrical steel, some patents (EP 0 540 405, EP 0 390 160) describe production processes in which the grain oriented electrical steel is produced, by means of secondary recrystallisation, starting from a directly cast strip (Strip Casting) and not from a hot rolled band. This kind of technology obviously leads to important economies in the production costs, in view of the production cycle simplification. However, due to the complexity of the secondary recrystallisation mechanism, to obtain a product of good magnetic characteristics a very strict control is necessary of the process parameters starting from the steel casting to the final annealing.
EP 0 540–405 discloses that to have a good quality of the product after the secondary recrystallisation it is necessary to produce in the solidified skin of the strip grains having the {110} <001> orientation, which is obtained by means of a quick cooling of the solidified skin in contact with the casting rolls, at a temperature of under 400° C.
EP 0 390 160 discloses that to have a good quality of the product, after secondary recrystallisation, it is necessary to control the strip cooling, in a first stage with a cooling rate of less than 10° C./s down to 1300° C., and then with a cooling speed of more than 10° C./s between 1300 and 900° C. By slow cooling down to 1300° C. a random texture of the cast strip is favoured, thus enhancing the formation of the desired {110} <001> grains, while the fast cooling between 1300 and 900° C. promotes the formation of fine second phases, able to act as inhibitors during the secondary recrystallisation.
Present inventors extensively studied the production of electrical steel by strip casting and found an alternative to the above patents, for the production of very high quality grain oriented Fe—Si. This new process, matter of present invention, is easy to control at an industrial scale and is able to give a product of good constant quality.